Demonstrators protesting cases of police brutality and the treatment of black Americans by law enforcement disrupted a presidential forum Saturday as O’Malley, a former Maryland governor, was interviewed on stage. The group later heckled Sanders.

The raucous scene unfolded when a large group of protesters streamed into the convention hall chanting, “Black lives matter!” As O’Malley and interviewer Jose Antonio Vargas looked on, one of the group’s leaders took over the stage and addressed the audience as the largely female group of demonstrators railed against police-involved shootings, the treatment of immigrants and Arizona’s racial history.

Before departing, O’Malley told the convention: “Black lives matter. White lives matter. All lives matter,” prompting some heckles and boos in the crowd.

He later apologized at an immigration event. “I meant no insensitivity by that and I apologize if that’s what I communicated,” he told reporters. “That was misstated. What I intended to say was that we’re all in this together — that black lives do matter and we have a double-standard of justice in this country.”

Sanders tried to address the roughly 3,000 Netroots activists as many of the protesters shouted at him and disrupted his remarks. At one point, Sanders said: “Black lives of course matter. I spent 50 years of my life fighting for civil rights and if you don’t want me to be here, that’s OK.”

During an abbreviated 20-minute appearance, the self-described democratic socialist from Vermont spoke about the need to address wealth and income inequality, noting that blacks and Hispanics face high rates of unemployment. Sanders was scheduled to meet with representatives of the organization Black Lives Matter after the speech, but his campaign manager, Jeff Weaver, attended the meeting instead, spokesman Michael Briggs said.

Sanders later addressed police brutality at a large rally Saturday night, telling a crowd of more than 11,000 it is unacceptable for young black men to be beaten and killed while walking down the street.

“When a police officer breaks the law, that officer must be held accountable,” Sanders said. He later quoted the 19th century black abolitionist Frederick Douglass: “Freedom doesn’t come without struggle.”

Sanders and O’Malley are vying to become the Democratic alternative to front-runner Hillary Rodham Clinton, who was campaigning in Iowa and Arkansas on Saturday and did not attend Netroots. Sanders has risen in polls in recent months and sought to broaden his appeal to minorities and a more diverse section of the Democratic electorate, addressing immigration and criminal justice.

Yet some liberal groups panned the appearances. Anna Galland, the executive director of MoveOn.org Civic Action, said the responses showed that all Democratic candidates have work to do to understand the black lives movement.

She added that while issues of economic and racial justice intersect, “portrayals of racial injustice as merely an offshoot of economic injustice or the implication that solutions to economic inequality will take care of racism represent a fundamental misunderstanding of how race operates in our country.”

The demonstrators were promoting the national “Black Lives Matter” movement, which seeks changes to law enforcement policies following several high profile deaths of black men at the hands of police.

Before the demonstrations, Vargas pressed O’Malley to defend his law enforcement record as Baltimore’s mayor a decade ago. O’Malley faced scrutiny when unrest broke out in his home city after an African-American man, Freddie Gray, died while in police custody in April.

O’Malley discussed his work to deal with violence and drug addiction in Baltimore, telling the crowd, “there are very few issues in our country that are quite as painfully intertwined as the legacy of violence, race and law enforcement in America.” He said his policies improved policing, reduced the number of police-involved shootings and increased drug treatment.

The Netroots Nation convention has been a rough place for presidential candidates before. Clinton last appeared in 2007, when it was called the Yearly Kos, and heard boos from some activists who disapproved of her vote for the 2003 invasion of Iraq and her acceptance of campaign contributions from lobbyists.

Clinton spoke up for raising the minimum raise during a dinner for state Democrats Saturday night in North Little Rock, Arkansas.

“Don’t forget, voters did come out and pass an increase in the minimum wage,” said Clinton, referring to the vote raising the minimum wage to $8.50 an hour by 2017. “Arkansas voters know paychecks need to grow, they know the economy is still stacked for those at the top. We just have to offer a plan for more growth and more fairness they can believe in and vote for.”